Oil shale is a naturally-occurring, shale-like rock which contains an organic component, usually referred to as kerogen, which upon heating releases volatile hydrocarbons which may be recovered as shale oil. A residual carbonaceous material typically remains with the inorganic component left following pyrolysis.
The pyrolysis or retorting process may be carried out in a retorting vessel of various designs. In one method, the raw oil shale is crushed and ground into particulate material which is fed into the top of the retorting vessel. The oil shale moves downward as a continuous bed of material through the retort. An upward flow of stripping gas countercurrent to the downward moving shale carries the volatile hydrocarbons away from the bed. See U.S. Pat. Nos. 4,199,432 and 3,167,494.
In such a retorting scheme, the countercurrent gas flow is usually sufficient to pneumatically entrain the finer particles of oil shale. Thus, the fine material entering the retort may become concentrated at the upper surface of the bed or be carried away with the stripping gas. The present invention is directed to a method for retorting oil shale without the disadvantages normally attendant to the presence of a fine shale fraction.